The present invention relates to a process for producing rubber-based graft copolymers having excellent powder characteristics. More specifically, the invention relates to a process for obtaining rubber-based graft copolymers in the form of substantially spherical fine particles from the latex thereof.
Hard polymers such as vinyl chloride resins, styrene resins, acrylonitrile-styrene resins, methyl methacrylate resins, polycarbonate resins and polyether resins have excellent stiffness, transparency and processability and, therefore, are widely used. However, since these hard polymers are brittle, the blending of a graft copolymer, which is obtained by polymerizing acrylonitrile and styrene or methyl methacrylate and styrene in the presence of a rubbery backbone polymer of the butadiene type, with the hard polymers is generally carried out to improve the impact resistance of the hard polymers. These graft copolymers are remarkably effective for improving the impact resistance of the hard polymers without impairing the inherent properties thereof and, therefore, are widely used.
These graft copolymers are generally prepared by an emulsion polymerization method. The desired graft copolymer is recovered by coagulating the resultant latex. More particularly, a large volume of an aqueous solution of an electrolyte (acid or salt) is poured into the latex, or conversely, the latex is poured into a large volume of the above-mentioned aqueous solution, to completely coagulate the latex, thereby forming a slurry. Then the slurry is dewatered and dried to obtain a graft copolymer in the form of a powdery resin.
The powder obtained by this method is, however, disadvantageous in that the constituent particles are irregular in shape and control of the particle size distribution is difficult. Furthermore, the powder contains a large amount of fine particles. As a result, the working environment becomes poor because of the dust, and the danger of dust explosion is increased. Furthermore, difficulties in handling the powder, such as a blocking phenomenon in which the powder particles adhere to each other during storage and process difficulty due to the poor fluidity of the powder, are liable to occur. Therefore, at the present time when reduction in labor is being sought by automation of powder weight measurement and the adoption of a large scale transportation system, there is a great demand for improvements in the powder characteristics, typically blocking resistance, fluidity and bulk density, with regard also to rubber-containing graft copolymers.